4 ms·

How does it compare to heatpumps, though? If a large-scale heatpump was used to power a district, it would only need 1/3 or even less of the electricty. From th

How does it compare to heatpumps, though? If a large-scale heatpump was used to power a district, it would only need 1/3 or even less of the electricty. From that perspective, does it make sense to store thermal energy or does a chemical battery become attractive again?

Moissanite 4y ago
I think the bigger problem with batteries is capex per kWh, rather than efficiency.
elil17 4y ago
You have to include the capex cost of the heat pumps. This design uses no equipment at the customer's homes, just pipes of hot fluid. Those pipes already exist in Finland, where district heating is relatively common. The next logical step is pumped thermal energy storage, where you pump heat into the sand using a heat pump - but there is significant capex associated with that.
- hedora 4y ago
  Heat pump efficiency drops rapidly with temperature differentials. This doesn't matter much when the goal is to heat air to room temperature, but it would matter a lot with a 600C target temperature. It's likely cheaper to use a lithium ion battery to store the electricity, then run a heat pump at the house off that. On top of that, it'll use a tiny fraction as much electricity.
  - elil17 4y ago
    It's true that heat pumps become less efficient at higher temperature differences. That's why these systems use lower storage temperatures. It's also true that, even then, the storage temperature has to by higher than the typical evaporator condensor for a heat pump, leading them to be less efficient (think 175% efficient vs. 300% efficient). You're looking at LCOS of $0.23 kWh-1 [1], which is higher than Li-ion. However, such a system may offer other advantages (better recyclability, no reliance on lithium, less risk of fire), or such systems may be cheaper in the future, which is why people are researching them. Additionally, these systems work with existing district heating pipes, which is great because it mean's you don't have to expand the grid capacity. In places with district heating, that could make a lot of sense. (I don't think anyone is proposing building new district heating systems, just retrofitting old ones). [1] https://www.sciencedirect.com/science/article/abs/pii/S019689042100248X https://www.sciencedirect.com/science/article/abs/pii/S01968... Also, Carnot efficiency increases without bound as the temperature difference decreases, so lowering the temperature of the room or increasing the temperature of the outdoors make a huge difference. The same applies vice versa for air conditioning which is why people use cooling towers.
zhynn 4y ago
Why not use heat pumps to heat the mass? What if the air conditioner exhausted the heat into your mass battery? It's not either/or.
- moffkalast 4y ago
  Well yeah that's what ground source heat pumps basically are. They just don't run at 600 degrees.
Schroedingersat 4y ago
Different purpose. A swimming pool sized silo of sand can heat a few hundred apartments or well insulated homes through winter using summer energy. Make it a few tens of metres high/deep and a handful will power a town. If there is going to be curtailment anyway then the energy is effectively free (or has negative cost). Or if you'd need to use something like pumped hydro or chemical storage then the heat pump only gives you a COP of 1-2 once you include storage losses.

Moissanite 4y ago

I think the bigger problem with batteries is capex per kWh, rather than efficiency.

elil17 4y ago

You have to include the capex cost of the heat pumps. This design uses no equipment at the customer's homes, just pipes of hot fluid. Those pipes already exist in Finland, where district heating is relatively common. The next logical step is pumped thermal energy storage, where you pump heat into the sand using a heat pump - but there is significant capex associated with that.

hedora 4y ago
Heat pump efficiency drops rapidly with temperature differentials. This doesn't matter much when the goal is to heat air to room temperature, but it would matter a lot with a 600C target temperature. It's likely cheaper to use a lithium ion battery to store the electricity, then run a heat pump at the house off that. On top of that, it'll use a tiny fraction as much electricity.
- elil17 4y ago
  It's true that heat pumps become less efficient at higher temperature differences. That's why these systems use lower storage temperatures. It's also true that, even then, the storage temperature has to by higher than the typical evaporator condensor for a heat pump, leading them to be less efficient (think 175% efficient vs. 300% efficient). You're looking at LCOS of $0.23 kWh-1 [1], which is higher than Li-ion. However, such a system may offer other advantages (better recyclability, no reliance on lithium, less risk of fire), or such systems may be cheaper in the future, which is why people are researching them. Additionally, these systems work with existing district heating pipes, which is great because it mean's you don't have to expand the grid capacity. In places with district heating, that could make a lot of sense. (I don't think anyone is proposing building new district heating systems, just retrofitting old ones). [1] https://www.sciencedirect.com/science/article/abs/pii/S019689042100248X https://www.sciencedirect.com/science/article/abs/pii/S01968... Also, Carnot efficiency increases without bound as the temperature difference decreases, so lowering the temperature of the room or increasing the temperature of the outdoors make a huge difference. The same applies vice versa for air conditioning which is why people use cooling towers.

zhynn 4y ago

Why not use heat pumps to heat the mass? What if the air conditioner exhausted the heat into your mass battery? It's not either/or.

moffkalast 4y ago
Well yeah that's what ground source heat pumps basically are. They just don't run at 600 degrees.

Schroedingersat 4y ago

Different purpose. A swimming pool sized silo of sand can heat a few hundred apartments or well insulated homes through winter using summer energy. Make it a few tens of metres high/deep and a handful will power a town. If there is going to be curtailment anyway then the energy is effectively free (or has negative cost). Or if you'd need to use something like pumped hydro or chemical storage then the heat pump only gives you a COP of 1-2 once you include storage losses.